The present invention relates to an arrangement for limiting the supply of fuel from a fuel pressure reservoir by way of a controllable injection valve into the combustion space of an internal-combustion engine. More particularly, the present invention relates to a controllable injection valve for injection into a combustion space of an internal-combustion engine, comprising a housing, an interior space formed in the housing having an inlet and an outlet, and a separating piston inserted in the interior space to divide the interior space into an inlet-side chamber and an outlet-side chamber in a liquid-tight manner. The separating piston is prestressed by a first spring against a fuel flow direction, and which, under the effect of forces applied thereto, is displacably arranged in a displacing direction between a starting position and an end position. A flow-through opening is formed in the separating piston to selectively connect the chambers with one another. The separating piston, at least in the end position, interrupts fuel flow-through. A displacement device is operatively arranged in the housing for closing the flow-through opening as a function of an axial displacing direction of the separating piston.
Known quantity limiting arrangements have different designs and methods of operation. Despite their constructionally different designs, however, these known arrangements all operate according to the same operating principles, specifically as a safety element during the occurrence of a disturbance in the fuel injection system and as a metering device in order to limit the quantity of injected fuel to a defined extent. If, for example, because of flow conditions, the injection valve does not close as intended, these quantity limiting arrangements separate the high pressure portion of the injection system from the injection valve and, therefore, prevent a continuous injection which may damage or even destroy the internal-combustion engine.
DE-OS 22 07 643 describes a valve in which the separating piston is displaced during the injection phase by a path corresponding to the injected amount of fuel against the force of the restoring spring. When the injection valve closes, the separating piston is moved back into its starting position under the effect of the restoring spring, while the fuel displaced in the process passes the piston through a piston duct.
When the injection valve does not close any further, in the known systems, a continuous injection of fuel is prevented by the piston while utilizing the pressure difference existing at the piston. Because, when then injection valve is closed, the fuel pressure existing on the output side at the piston is comparatively low, and, on the input side, the constantly high injection reservoir pressure acts upon the separating piston, the piston is pressed in the direction of and against the piston stop. In this position, the separating piston seals off the outlet of the quantity limiting arrangement and thus prevents a fuel injection.
Although the known quantity limiting arrangement prevents a continuous injection in the case of high system pressures, it does not offer sufficient protection against a continuous injection in operating ranges with a reduced system pressure of 100-300 bar, as occur, for example, during the starting operation and during the starting phase of the internal-combustion engine. Then, this known arrangement is particularly disadvantageous because, for example, by a corresponding dimensioning of the piston restoring spring, it is adapted to high system pressures and, when the system pressure is reduced, because of the small pressure difference occurring at the separating piston, the piston is not activated or is not activated sufficiently. For this reason, fuel will flow through the piston duct into the combustion space in an unhindered manner.
Injection quantity limiting arrangements are required particularly in fuel injection systems in which the fuel is continuously acted upon by a high pressure in a pressure reservoir and is injected into the combustion space from this pressure reservoir by a correspondingly controlled opening of injection valves constructed especially for such an accumulator charging. DE 31 19 050 A1 describes such a quantity limiting arrangement which is arranged as a safety valve in the housing of an injection valve between the accumulator space and the seat surface for the nozzle needle. A movable part of this arrangement, which is normally held in the open position by a spring, can be loaded by way of a piston with the fuel pressure in the accumulator space so that the movable part arrives in the closed position. This safety valve is also adapted to high system pressures so that, when system pressures are reduced, the pressure energy will not be sufficient for overcoming the restoring force of the spring. An interruption of the fuel flow is therefore not ensured in the case of such a system operation. In addition, a separate change-over valve is required for the described safety valve in order to control the pressure admission to the piston. As a result, this quantity limiting arrangement, which consists of the change-over valve and the safety valve, requires high expenditures and costs.
Furthermore, the above-described known injection valve and other injection valves suitable for accumulator charging have a number of throttling points and control ducts which is higher than in the case of conventional injection valves, which renders these valves more susceptible to disturbances with respect to dirt particles or chips carried along in the fuel flow. Because even small suspended particles may prevent a clean valve closure, the connected risk is relatively high of a continuous fuel injection into the combustion space and finally of damage to the internal-combustion engine.
An additional disadvantage of the known valve is that no control or indicating possibilities are provided by way of which the actual operating condition of the quantity limiting arrangement and/or the failure or an occurring disturbance of an injection valve can be indicated or determined.
It is, therefore, an object of the present invention to provide an arrangement for limiting the maximally possible quantity of fuel injected into the combustion space which, independently of the operating condition, permits a reliable limiting of the maximal quantity of injected fuel and which, as a safety element, protects the engine from possible damage.
This object has been achieved in accordance with the present invention by providing a displacement device such that the flow-through opening is closeable as a function of the displacing direction of the separating piston.
One essential advantage of the arrangement according to the present invention comprising a separating piston and a closing device is the resulting liquid-tight separation of the high-pressure portion of the injection system with respect to the injection valve in the starting condition, during the injection phase and possibly beyond that during the whole period in which the injection valve is switched into its open position. This separation of the fuel supply to the injection valve which is created as a function of the displacing direction of the separating piston has the effect that, even in the case of the smallest pressure differences applied to the separating piston, no fuel can flow past the separating piston. As a result, even in the case of a malfunctioning of the injection valve in low-pressure ranges during the starting phase of the internal-combustion engine, the separating piston is displaced to the right stop and therefore reliably prevents continuous injections.
With the arrangement of the present invention, the fuel supply and the fuel metering are uncoupled. During the restoring stroke, while the closing device is open, the fuel quantity corresponding to the respective separating piston stroke is displaced through the flow-through opening into the outlet-side chamber. This arrangement ensures that, independently of the fuel quantity injected during the preceding injection phase, at the beginning of each injection, the constructively determined maximal fuel supply quantity is available in the outlet-side chamber. The fuel injection and the fuel metering therefore take place in successive time periods and each of the functions can be optimally adapted to the different variables which determine those functions.
When the injection valve is open, while utilizing the high fuel pressure existing on the high pressure side on the separating piston, a supply of fuel to the injection valve takes place which is as unhindered as possible. Irrespective of the duration of the injection phase, the fuel quantity is maximally injected which was constructively provided by the volume of the outlet-side chamber (i.e. with the piston in the starting position).
The fuel afterflow into the outlet-side chamber is advantageously provided in the longer time period of the restoring stroke of the separating piston. Because the injection valve is closed during the restoring stroke, the previously injected fuel quantity can be redelivered without having to consider the desired injection course inasmuch as the injection course is determined by the working stroke of the separating piston.
Special advantages are achieved if the closing device is constructed in the housing interior in a self-regulating manner without any external control because this provides an operation of the device which is independent of disturbing outside influences and results in particularly low component expenditures and, therefore, in a particularly low susceptibility to disturbances.
Up to now, an additional fuel filter was connected, as a measure against the dirt-caused susceptibility to disturbances of used injection valves, directly in front of the injection valve into the fuel pipe and keeps back the suspended substances. Because during the injection, large fuel mass flows flow through the filter, as a result of the system, conventional filters had to have very large dimensions so that the filter will cause no additional throttling point in the inflow system which may impair the course of the injection. In one currently preferred further embodiment of the quantity limiting arrangement according to the invention, the fuel filter is combined with the separating piston. This filter is inserted into a correspondingly constructed receiving space in the separating piston. As a result, the filter cross-section may be sized much smaller than fixedly installed conventional filters because, during the piston restoring stroke, thus in the phase which is between two injections, fuel will flow through the filter. In this phase, the filter-caused throttling effect plays a subordinate role. Furthermore, occurring throttling losses may simply be compensated by a correspondingly strong dimensioning of the piston restoring spring.
If the fuel filter is gradually closed by dirt particles, when conventional filters are used, the corresponding filter was not supplied with the fuel quantity corresponding to the operating requirements of the engine, but, with a considerably impaired efficiency, was operated until either the filter was completely clogged or the engine was switched off because of a noticeable operating behavior. In contrast, the filter according to the present invention which is inserted into the separating piston advantageously indicates its own maintenance intervals. When this filter reaches a degree of contamination which impairs the flow-through of fuel, a fuel back pressure is generated in front of the filter which equals an inflow-side pressure increase. Because of the resulting pressure compensation weight on the separating piston, this separating piston is displaced correspondingly more and farther toward its closed position. In such a system compensation, the quantity limiting arrangement will close before the cylinder is insufficiently supplied with fuel. This means therefore that, starting from a certain contamination of the filter, the cylinder arranged behind it is switched off before it is supplied in an uneconomical manner. A delaying or exceeding of the filter servicing intervals is therefore impossible. After the cleaning of the filter, the engine can continue to be operated normally.
A further currently preferred embodiment of the present invention provides a device for determining the separating piston position. The desired optimized injection course is not disadvantageously influenced by the device according to the invention, although, by way of an additional monitoring and control device, the protection against continuous injections and connected engine damage can be increased significantly.
The above considerations were carried out in the manner of an example by reference to a one-cylinder engine. In practice, in the case of an internal-combustion engine, the complicated interplay of several cylinders must be mutually coordinated. When considering the entire internal-combustion engine, control-caused, as well as construction-caused, differences occur from one cylinder to the next. For optimization of this interplay of all cylinders of an internal-combustion engine, it is therefore of principal significance to detect the fuel supply as comprehensively and precisely as possible. Based on these monitored operating data, possibly occurring deviations and disturbing influences can be correspondingly controlled by the engine electronic system.
A main advantage of the device for determining the piston position according to the present invention is that the respective momentarily injected fuel quantity can be determined by a path signal which can be measured relatively easily with the constructively determined, known geometrical dimensions. In addition, information is obtained directly from the path measurement concerning the start and the end of the injection. Furthermore, the breakdown of an injection valve dan be determined without any time delay which had been impossible up to now.
A further embodiment of the piston path measuring device according to the present invention as a no-contact position measuring device has the advantages of no-wear measuring devices and resulting low susceptibilities to disturbances which lead to a correspondingly long service life. In yet another currently preferred embodiment of the invention, an inductive path measuring device is used for this purpose whose components are available as standardized parts.
For a simple, low-cost construction of a measuring device, it is suggested to mount a signal generator, for example, a conductor coil, on the outer circumference of the housing, for the purpose of which the housing is constructed of non-magnetic materials, but the separating piston is constructed of magnetic materials.